NASA has repeatedly changed estimates on whether a huge Big Ben-sized asteroid first spotted last year will crash into Earth - and now an expert has revealed the reason why.
The world breathed a sigh of relief on Tuesday after the agency announced the chances of the YR4 hitting us
Nasa said the asteroid has just a 1-in-26,000 chance of striking Earth - meaning there is a 99.9961 per cent chance the rock will sail past.
YR4 was previously estimated to have an alarming 1-in-32 chance of smashing into us.
Richard Binzel, a Professor of Planetary Science at MIT, revealed to The Sun how the odds dramatically flipped to a "Level 0" threat.
He said: "When we first discover an asteroid, we see only a tiny piece of its orbital path.
"Astronomers always check immediately to make sure that orbital path isn't intersecting with Earth any time soon.
"The scientific challenge comes when that initial orbit check doesn't immediately rule out an impact course."
This is what happened with YR4 and as astronomers began gathering and updating the data, the cycle of " initial and ever-changing probabilities" began.
"It is not unusual for the probability numbers to bounce around a bit, simply because the amount of data in the first few weeks and months is usually quite limited," Binzel said.
Eventually, more data revealed came in about the asteroid's orbital path which flipped
"Those probability numbers became vanishingly small," Binzel said, as scientists saw that the asteroid would most likely entirely miss our planet.
"Once we fall below 1-in-1000 probability, we categorize the object as Level O on the Torino Impact Hazard Scale," he added.
Binzel invented the Torino scale - the system used since 1999 to categorize potential Earth impact events and he explained that Level 0 means there is "No Hazard".
"[It] is effectively the "All Clear Signal" for paying detailed attention to the object," he said.
"In other words, we now have enough data to be completely secure that this particular asteroid doesn't pose any future threat to us."
Astronomers first noticed that the asteroid was hurtling towards Earth in December - and it shot straight to the top of the risk list.
YR4 had previously been graded as level three on the Torino scale.
This emergency level means: "A close encounter, meriting attention buy astronomers. Current calculations give a one per cent or greater chance of collision capable of localised destruction."
The odds for YR4 striking Earth began at a 1-in-83 chance, before reducing to 1-in-67, 1-in-53, 1-in-43, 1-in-38, 1-in-32 and then to 0.
If YR4 - estimated between 40m and 90m wide - did strike Earth, it would pack a punch scientists calculated.
It would blow a 1.2mile-across crater into the Earth's crust, and might look like.
Physicists cutting across some of the world's most populated areas, including Bogotá, Abidjan, Lagos, Khartoum, Mumbai, Kolkata, and Dhaka.
But now scientists say there will be 167,000 miles of clear sky between the rock and our planet - and no Christmas collision.
The space rock will appear near Earth again in 2028 - around 5 million miles away - giving scientists another chance to gather data on it.
It will then reappear on December, 22, 2032 which was the approach causing such alarm.
Boffins are now excitedly looking into how technological advancements can help them study asteroids and their orbital paths.
New images of YR4 taken on February 7 by the Gemini South Telescope in Chile showed the rock in the clearest detail yet.
They showed 2024 YR4 as a bright smudge in space —captured while it was 37 million miles away.
Bryce Bolin, a Nasa astronomer involved in capturing them said: "Only a few asteroids have been studied like this."
Binzel said: "These objects are there, and have always been there.
"Now we're advancing the technology to reveal them into plain sight out of the darkness.
"So rather than it being worrisome when we find them, it is actually a triumph for technology that we can find and check them off as being one less object that could ever take us by surprise."
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